This invention relates to an apparatus and process for making paper and it particularly relates to an apparatus and process for continuously moving paper being processed along a preselected and desired path of travel while avoiding deviation of the paper from the desired path of travel; and more specifically, the invention herein relates to a closed draw type of paper making machine.
Two basic types of paper transfers made in conventional paper making machines are the "closed draw" type and the "open draw" type. Generally speaking, the closed draw system, involved in the present invention, is one wherein a nip is defined between two contacting members, such as an upper felt and a lower felt, a forming wire and a felt, or a felt and a Yankee dryer. The closed draw type of paper transfer is in contrast to the open draw type, not involved herein, which has the paper pass across an open space from one paper contacting member to another such member, and no nip is defined between the members.
Problems encountered with transferring paper across an open space in the open draw system is quite different from problems encountered when transferring paper between two contacting members defining a nip in a closed draw system. Simply stated, in the open draw system, once the paper has been transferred from one member to the other, the paper generally remains in contact with the receiving member without difficulty. However, in the closed draw system, problems are encountered in maintaining the paper in the desired path of travel becuase the paper sometimes tends to be carried with the wrong contacting member after passing through the nip defined between the contacting or processing members. The invention herein relates to improvements in the transfer of paper in paper making machines of the closed draw type, as distinguished from the open draw system.
In the processing of paper to make various types of paper materials, such as "wet" or "dry" crepe tissue paper, toweling, and the like, the paper processing is conventionally performed on various types of Fourdrinier paper making machines. In a conventional Fourdrinier paper making machine, a slurry of pulp fiber, at a concentration of about 0.2 to 0.6% is placed or squirted on a forming wire at the slice section of the machine. Water immediately begins to drain from the pulp by gravity through the forming wire. The drainage is assisted by the action of various water removal devices, such as a suction breast roll, forming board, foils, rolls, suction boxes and suction hip roll. The paper is carried to a first nip between the forming wire and the top pick-up felt. The pick-up felt is backed up by a "kiss" pick up roll, a shoe, or a suction pick-up roll. At this nip or contact point, the paper sheet is transferred from the wire to the pick-up felt. The pick-up felt carries the paper sheet on its underside to the press section of the machine, where more water is removed from the paper. At the press section, paper is normally carried by the pick-up felt between a top press and a bottom press roll; although depending on the particular machine, no press section may be used in processing certain types of paper. Following passage through the nip of the press, the paper sheet must stay with the top pick-up felt and must not transfer to the bottom felt.
Following the press section, the paper sheet is carried by the top pick-up felt to the nip defined between the first pressure roll and the hot, smooth surface of the Yankee dryer. At this nip, the paper is transferred from the pick-up felt to the Yankee dryer surface. Following this, the paper sheet may be compacted or pressed between a second pressue roll and the Yankee dryer. The paper is carried on the Yankee dryer to a doctor blade which crepes and removes the self-sustaining paper from the surface of the Yankee dryer. The paper is thereafter either wound up on a reel or passed around after-dryers and then wound up on a reel.
From the foregoing, it is seen that there are at least three times, during normal paper processing or making, when the paper sheet must pass between a pressure point or nip between contacting members. In the event that, if during passage through any of these pressure points, the paper sheet does not remain in the desired and preselected path of travel, production will be interrupted and there may be resulting damage to the equipment. It is thus very important to minimize the risk of production stoppage and/or machinery damage becuase of the significant expenses which might result from production shutdown and machinery damage.
There are several theories as to why a paper sheet being processed does or does not follow the desired preselected path of travel. One theory as to why the paper sheet moves in the desired path of travel is that following passage through a nip, the paper sheet follows the smoother of the two contacting members at the outgoing or discharge side of the nip. In this regard, at the "kiss" pick-up point, the contacting members are the forming wire and the pick-up felt. Since the surface of the woven pick-up felt is smoother than the relatively rough surface of the forming wire, the paper sheet leaves the forming wire and follows the felt. In fact, it has been found that, by wetting the surface of the top pick up felt with water, the felt surface becomes even smoother so as to facilitate the proper transfer of the wet paper sheet from the forming wire to the pick-up felt.
As a further example of the above theory, when a bottom felt is used in the paper making machine, the bottom felt is intentionally less smooth than the top pick-up felt; since it is not as smooth as the top pick-up felt, the paper remains with the smoother top felt at the outgoing side of the nip.
Although, in theory, paper should continue movement in its desired and preselected path of travel, in both of the above-mentioned conditions, at the transfer point between the wire and the pick up felt, other problems may occur. At high speeds and unless the pick-up felt surface is wet with water to provide a smooth surface, or if the paper sheet is pressed into the mesh of the forming wire, the paper may not properly transfer to the pick-up felt and production may be interrupted. In the press section, although the bottom felt is intentionally maintained less smooth than the pick-up felt, when a new pick-up felt is installed and the bottom felt is old and worn smooth, there is sometimes a tendency for the paper sheet to follow the bottom felt. When this occurs, it is not unusual for the bottom felt to be replaced even though there is a significant amount of use remaining in the bottom felt.
At the nip between the hot smooth Yankee dryer surface and the top pick-up felt, it might be expected that there would be no problem of transfer from the pick-up felt to the Yankee dryer surface, because the Yankee dryer surface is very smooth as compared to the pick-up felt. However, there is a tendency for the cold paper sheet to blister from the hot Yankee dryer surface when the wet sheet touches the surface. Another theory as to why the wet paper sheet sometimes tends to blister from the hot Yankee dryer surface is that a negative pressure is produced at the outgoing or discharge side of the nip. This negative pressure reduces the boiling point of the water, thereby increasing the tendency of the sheet to blister from the hot surface and to undesirably follow the top felt and interrupt production.
In the case of heavy and well closed paper sheets, the blistering problem is accentuated and sometimes it is necessary to make a compromise between various parameters, including the temperature of the Yankee dryer surface, amount of pulp refining, and the speed of the machine while the sheet is formed so that the machine will run safely without any interruptions. Any such compromise, however, has the obvious disadvantage of creating a production slowdown and of compromising quality.
Prior art patents showing various paper processing machines, both of the closed draw type and of the open draw type, are Pope U.S. Pat. No. 1,338,094; Monaghan U.S. Pat. No. 1,581,656; Goodwillie U.S. Pat. No. 2,694,346; Dearden U.S. Pat. No. 2,780,967; Nykopp U.S. Pat. No. 3,655,507; Douglas et al U.S. Pat. No. 3,560,333; Millspaugh U.S. Pat. No. 1,163,252; Rance et al U.S. Pat. No. 2,990,013; Murray et al U.S. Pat. No. 3,351,521; British Pat. No. 812,914; and Canadian Pat. No. 452,200. A number of these patents recognize problems encountered in transferring paper and in maintaining the paper in the desired path of travel.
For example, the Pope patent shows the use of an air blast to assist in transferring paper in an open draw transfer system. Also, the Monaghan patent shows the use of air pressure applied on the underside of the forming wire to separate the paper from the forming wire, but again, in an open draw system, the type of paper transferring not involved herein.
The Goodwille patent relates to the transfer of paper between two processing members wherein a "dandy roll" bridges the gap between two of the processing members. The Goodwille patent recognizes the differences between closed draw and open draw systems and seems to suggest a "hybrid" system by using the "dandy roll" and air blasts in an open draw system.
The Dearden patent shows a closed draw system using an air blast to assure initial transfer of the "tail" from the forming wire only. When the tail has been transferred, the air supply to the blow roll is cut off. The air blast is applied on the ingoing side of the nip.
The Nykopp broadly shows a closed draw system, wherein a blow roll, on the incoming side of the nip, introduces heated air to the space before the press nip to maintain or increase the differential pressure so as to enhance water removal.
The Douglas et al patent uses steam applied to the surface of the paper web prior to the nip between that suction roll and the Yankee dryer. The Millspaugh patent shows an open draw system, which uses air pressure pipes inside of a suction roll to help the paper transfer. The Rance et al patent shows an open draw system, wherein at the point where the paper leaves the forming wire, a box is provided with suction and pressure slots therein.
The Murray patent again shows an open draw system with air being blown against the lower side of the wire to loosen the web prior to transfer.
The British patent shows an open draw system using an air blast. The Canadian patent shows a press section with an pneumatically loaded hood to apply pressure to the paper pulp.